646 FUNDAMENTALS OF FRUIT PRODUCTION 



The point should be emphasized that the amount of air drainage 

 secured by selecting a site somewhat above the adjoining fields depends 

 not alone on the amount of elevation, but also on the area from which 

 the cold air drains in comparison with the extent of that to which it 

 may settle. If the low ground upon which the cold air may sink is 

 limited in extent and has little outlet while the area to be drained is 

 large, this depression will soon be filled with cold air and the slope above 

 will be afforded no further protection. The case is comparable with a 

 large watershed supplied with an inadequate drainage system. An 

 elevation of 20 or 25 feet above a wide valley may thus afford better 

 air drainage for one orchard than an elevation of 50 feet above a narrow 

 valley affords another. In many cases a ravine or narrow draw along one 

 side of an orchard will afford a given site better air drainage than an 

 adjoining low-lying field covering many acres, provided the draw or 

 ravine is deep, has a good outlet and is not clogged with brush and timber 

 that interferes with free movement of the air. In other words, of 

 two areas having the same elevation one may enjoy much better air 

 drainage and greater freedom from frost because of differences in the 

 contour and topography of the land that borders them. 



The graphs in Fig. 68 show the maximum variations in temperatures 

 during the night between stations at different elevations on a hillside. 

 Though day temperatures are not given there is the suggestion that 

 they approximate rather closely. Available data show that such in- 

 equalities in elevation as are normally found within single fruit growing 

 districts are responsible for but small differences in maximum day 

 temperatures. 2 In other words, elevation materially influences minimum 

 and average, but not maximum, temperatures. 



Thermal Belts. — The influence of elevation on air drainage and 

 consequently on the selection of sites for fruit growing should not be 

 passed over without a reference to the so-called "thermal belts," "ther- 

 mal zones," "frostless belts" or "verdant zones," as they are variously 

 called. They are comparatively frost-free belts along hillsides or mountain 

 ranges, below and above which frost occurrence is not uncommon. The 

 limits of comparatively few such zones have been accurately mapped; 

 consequently the fruit industry has developed more or less independently 

 of them. However, their occurrence presents an interesting phenomenon 

 and it is desirable to recognize and if possible, make use of the obvious 

 advantages they provide, for without doubt the fruit growing districts 

 of the country include many such zones that are not being utihzed for 

 fruit production. 



The following quotations from an article by Abbe^ will point out 

 more exactly the conditions characteristic of thermal belts : 



"Prof. J. W. Chickering, Jr., in the Bulletin of the Philosophical Society of 

 Washington, March, 1883, and in the American Meteorological Journal, Vol. I, 



